(well ... that was an offtopic comment (the web layout ... sometimes makes the text controls jump to the next line, and some glitches like that ...))

I'm still performing some test-and-error efforts and solved by myself some questions but ... i still don't know the purpose of "special_type" node in defaultvalures.rows.row nodes . What's that node used for?

The "+SGQA" format is used in Multiple-choices questions in order to refer to a single checkbox.
When the "SGQ" code is used for a multiple choices question, then the condition applies to the checkbox group.